UNIVERSITY  OF   CALIFORNIA  PUBLICATIONS. 


COLLEGE  OF  AGRICULTURE, 


AGRICULTURAL  EXPERIMENT  STATION. 


THE  HOP  APHIS. 


By  WARREN  T.  CLARKE. 


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Hop  Yard,  showing  male  hop  plants  and  supply  wagon. 


BULLETIN    No.    160. 

(Berkeley,  Cal.,  June,  1904.) 


SACRAMENTO: 

W.    W.    SHANNON,     III'.      I     SUPERINTENDENT   STATE   PRINTING, 

1904. 


BENJAMIN  IDE  WHEELER,  Ph.D.,  LL.D.,  President  of  the  University. 

EXPERIMENT  STATION  STAFF. 

E.  W.  HILGARD,  Ph.D.,  LL.D.,  Director  and  Chemist. 

E.  J.  WICKSON,  M.A.,  Horticulturist. 

W.  A.  SETCHELL,  Ph.D.,  Botanist. 

ELWOOD  MEAD,  M.S..  C.E.,  Irrigation  Engineer. 

C.  W.  WOODWORTH,  M.S.,  Entomologist. 

R.  H.  LOUGHRIDGE,  Ph.D.,  Agricultural  Geologist  and  Soil  Physicist.    (Soils  and  Alkali.) 

M.  E.  JAFFA,  M.S.,  Assistant  Chemist.    (Foods,  Nutrition.) 

G.  W.  SHAW,  M.A.,  Ph.D.,  Assistant  Chemist.    (Starches,  Oils,  Beet-Sugar.) 

GEORGE  E.  COLBY,  M.S.,  Assistant  Chemist.    (Fruits,   Waters,  Insecticides.) 

RALPH  E.  SMITH,  B.S.,  Plant  Pathologist. 

A.  R.  WARD,  B.S.A.,  D.V.M.,  Veterinarian,  Bacteriologist. 

E.  H.  TWIGHT,  B.Sc,  Diploma  E.A.M.,  Viticulturist. 

E.  W.  MAJOR,  B.Agr.,  Animal  Industry. 

A.  V.  STUBENRAUCH,  M.S.,  Assistant  Horticulturist,  in  charge  of  Substations. 

WARREN  T.  CLARKE,  B.S.,  Assistant  Field  Entomologist. 

H.  M.  HALL,  M.S.,  Assistant  Botanist. 

H.  J.  QUAYLE,  A.B.,  Assistant  Entomologist. 

GEORGE  ROBERTS,  M.S.,  Assistant  Chemist,  in  charge  Fertilizer  Controt 

C.  M.  HARING,  D.V.M.,  Assistant  Veterinarian  and  Bacteriologist. 

C.  A.  TRIEBEL,  Ph.G.,  Assistant  in  Agricultural  Laboratory. 

C.  A.  COLMORE,  B.S.,  Clerk  to  the  Director. 


EMIL  KELLNER,  Foreman  of  Central  Station  Grounas. 

JOHN  TUOHY,  Patron,  ) 

{r  Tulare  Substation,  Tulare. 
JULIUS  FORRER,  Foreman,  ) 

J.  E.  McCOMAS,  Patron,  Pomona, 

J.  W.  MILLS,  Superintendent,  Ontario,  }■  Southern  California  Substation. 

JOHN  H.  BARBER,  Assistant  Superintendent,  Ontario, 

A.  A.  KNOWLTON,  Patron, 

J.  H.  OOLEY,  Workman  in  charge, 

ROY  JONES,  Patron,        ) 

,.r„    ~^TT „  f   University  Forestry  Station,  Santa  Monica. 

WM.  SHUTT,  Foreman,    \ 

H.  O.  WOODWORTH,  M.S.,  Foreman  of  Poultry  Station,  Petaluma. 


>  University  Forestry  Station,  Chico. 


The  Station  publications  (Reports  and  Bulletins),  so  long  as  avail- 
able, will  be  sent  to  any  citizen  of  the  State  on  application. 


THE  HOP  APHIS. 

(Phorodon  humuli,  Schrank.) 


By  WARREN  T.  CLARKE. 


In  certain  of  the  hop-growing  sections  of  California  the  hop  aphis 
is  at  times  a  serious  menace  to  the  industry.  Owners  of  hop  yards 
dread  the  appearance  of  these  minute  insects  on  their  vines,  well  know- 
ing that  if  they  increase  unchecked  great  losses  will  result.  Indeed, 
instances  are  known  in  this  State  where  the  value  of  the  hop  crop  has 
been  reduced  fully  one  half,  because  of  the  presence  of  the  aphis.  It 
is  known  and  feared  in  all  parts  of  the  world  where  hops  are  grown, 
and  entomologists  on  the  continent  of  Europe,  in  England,  and  in  this 
country  have  devoted  much  study  to  this  pest.  Studies  of  the  hop 
aphis  here  in  California,  however,  seem  to  show  that  the  results  obtained 
in  these  other  sections  do  not  fully  apply  here,  owing  probably  to 
climatic  differences.  This  will  be  more  fully  brought  out  in  the  course 
of  this  discussion  of  the  insect  and  its  activities  in  this  State. 

Spring  Appearance  and  Distribution.— Under  the  California  condi- 
tions of  climate  and  soil  the  first  hop  plants  to  begin  growth  in  the 
spring  are  those  bearing  staminate  flowers  only,  the  males  (variously 
called  "He  Hops,"  "Bulls,"  "Los  Toros,"  etc.).  Leaves  and  runners 
appear  upon  these  from  ten  days  to  two  weeks  before  growth  begins 
with  the  female  or  pistillate-flowered  plants  from  which  hops  are 
gathered,  and  they  remain  green  for  some  time  after  these  female 
plants  have  become  dry  and  unsucculent.  The  staminate-flowered  or 
male  plants  are  scattered  about  the  hop  yard  usually  in  the  proportion 
of  one  of  these  to  from  one  hundred  and  fifty  to  two  hundred  of  the 
pistillate  plants,  and  by  them  pollination  of  the  hop  is  accomplished. 
(See  Fig.  1  and  Frontispiece.)  It  seems  that  in  the  hop  yards  of  this 
State  the  aphids  invariably  appear  first  upon  the  under  side  of  the  leaves 
of  the  male  plants,  and  they  can  usually,  in  affected  fields,  be  seen  upon 
them  from  two  to  three  weeks  before  any  can  be  discovered  upon  the 
female  plants.  On  May  2,  1903,  we  found  upon  the  leaves  of  male  hop 
plants  in  the  Pajaro  Valley  wingless  parthenogenetic*  female  hop 
aphids  and  their  young.  In  one  instance  the  mother  aphid  had  clustered 
about  her  seven  of  her  offspring.     The  plant  upon  which  the^e  were 


*Insects  propagated  without  sexual  reproduction. 


4  UNIVERSITY  OF  CALIFORNIA— EXPERIMENT  STATION. 

found  had  grown  out  about  one  foot  and  the  aphid  colony  was  upon  the 
lowest  leaf.  On  the  date  in  question  and  on  the  following  day  (May  3, 
1903)  a  number  of  hop  plants,  invariably  staminate  ones  (for  indeed 
the  pistillate  plants  had  not  yet  begun  to  grow),  were  found  to  have 
aphids  upon  the  lower  leaf  or  leaves.  Inquiry  developed  the  fact  that 
in  the  yards  under  observation  the  attack  had  always  previously  begun 
in  the  localities  where  these  were  found,  the  infection  spreading  from 
these  points  until  finally  large  areas  of  the  yards  were  affected.  These 
starting  points  of  the  hop-aphis  attack  in  these  yards  in  1903  were  care- 
fully noted,  and  the  development,  and  course  of  the  trouble  observed 
through  the  year.  In  from  fourteen  to  twenty-one  days  after  the  first 
wingless  parthenogenetic  female  aphids  were  observed,  an  occasional 
winged  female,   also  parthenogenetic,  was   developed.     These  winged 


Fig.  1.    General  view  of  hop  yards,  showing  male  plants. 

aphids  passed  to  the  female  hop  plants  and  the  infection  was  thus 
spread  pretty  generally  through  the  yards.  By  the  time  the  hops  began 
to  form  the  aphids  were  very  numerous  and,  where  no  remedial  work 
was  undertaken,  the  damage  done  by  them  was  very  considerable. 

Character  of  Injury.— The  injury  wTought  by  the  aphids  upon  the 
pistillate  plants  (female  hops)  is  twofold.  At  first  the  attack  is  con- 
fined to  the  leaves  and  tender  growing  tips  of  the  shoots,  and  the  size 
of  the  leaves  of  the  plants  when  the  lice  are  numerous  upon  them  is 
greatly  reduced.  So  severe,  indeed,  was  the  effect  upon  the  plants 
under  observation  that  by  the  first  of  July  attacked  vines  in  the  yards 
could  be  easily  distinguished  from  the  plants  that  were  not  attacked, 
because  of  the  smallness  of  the  leaves  and  their  yellow,  dry  appearance. 
The  crop  upon  these  affected  plants  is  of  small  size  and  light  weight 
and  greatly  reduced  in  value.    A  more  serious  injury,  however,  is  that 


THE  HOP  APHIS.  5 

to  the  hop  cones  themselves,  because  of  the  direct  attack  of  the  aphids  on 
the  heads.  The  insects  seem  to  find  the  young,  newly-forming  cones 
very  much  to  their  taste  and  gather  in  great  numbers  in  them,  generally 
at  the  base  of  the  bracts.  When  the  hops  are  gathered  and  sent  to  the 
kiln  for  drying,  these  aphids  remain  in  place,  and  the  result  is  that  the 
finished  product  is  of  poor  quality  and  aroma  because  of  the  dried 
bodies  of  the  insects  in  them.  This  reduces  not  alone  the  value  of  the 
individually  affected  hops,  but  also  of  the  whole  lot  in  which  they  may 
occur.  The  total  injury  determined  by  comparing  yards  that  were 
similar  in  every  respect  of  soil,  climate,  and  exposure,  was  such  that  the 
crop  of  the  yards  where  no  control  work  was  attempted  and  where  the 
aphids  were  allowed  to  take  their  full  course,  returned  to  the  owners  not 
more  than  one  half  as  much  per  acre  as  did  those  yards  where  control 
work  was  done. 

EXPERIMENTS   WITH   REMEDIES. 

While  the  hop  aphids  appeared  in  the  Pajaro  Valley  yards  very  early 
in  May,  1903  (first  observed  May  2),  they  did  not  become  numerous 
enough  to  constitute  a  serious  menace  to  the  crop  until  the  middle  of 
June.  At  this  time  they  had  become  quite  well  distributed  from  the 
points  of  beginning,  and  were  even  occasionally  to  be  found  in  the 
newly-forming  hops ;  and  a  brisk  campaign  was  organized  against  them. 
Through  the  co-operation  and  assistance  of  certain  growers  of  hops  in 
the  region  we  were  enabled  to  carry  out  a  series  of  experiments  in 
spraying  on  two  hop  yards.  This  work  resulted  in  a  complete  control 
of  the  aphids,  and  very  greatly  increased  the  value  of  the  yield  in 
these  yards  over  that  of  the  adjacent  yards  that  were  not  treated. 

Kerosene  Emulsion  and  Tobacco.  —  One  yard,  comprising  some  forty 
acres,  is  situated  near  the  town  of  Watsonville,  in  the  Pajaro  Valley. 
During  the  third  week  in  June  the  attack  of  the  aphids  in  this  yard 
became  so  serious  that  it  seemed  as  though  a  large  proportion  of  the 
crop  would  be  ruined  by  them.  The  male  vines  in  certain  portions  of 
the  yard  were  very  much  infested  with  the  aphids,  and  they  could  also 
be  found  in  goodly  numbers  on  the  female  vines  and  even  in  the  young 
hops.  After  some  minor  experimenting  to  decide  the  killing  power  of 
the  material  and  its  effect  on  the  hop  foriage,  we  decided  to  treat  this 
yard  with  a  spray  made  up  of  tobacco  decoction  and  kerosene  emulsion 
in  combination.  The  tobacco  decoction  was  made  by  steeping  tobacco 
stems  and  refuse  from  cigar  factories  for  from  two  to  three  hours  in 
water  that  was  kept  heated  to  just  below  the  boiling  point.  One  pound 
of  tobacco  refuse  was  used  to  each  two  gallons  of  water,  and  by  this 
steeping  process  a  quite  strong  tobacco  juice  resulted.  The  kerosene 
emidsion  was  made  by  dissolving  seven  and  one-half  pounds  of  ordinary 


b  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION. 

laundry  soap  in  fifteen  gallons  of  hot  water,  and  to  this  adding  five 
gallons  of  kerosene  oil.     The  soapy  water  and  the  oil  were  thoroughly 


Fig.  2.    Spray  outfit. 


churned  together  for  from  fifteen  to  twenty  minutes.  This  was  best 
done  by  pumping  back  the  material  on  itself  through  the  spray  nozzle. 
The  result  was  a  fairly  stable  emulsion  of  a  creamy  consistency.     We 


Fro.  :?.    Spray  outfit, 

found  it  best  not  to  make  more  of  the  tobacco  decoction  than  could  be 
used  up  in  two  days,  because  if  the  material  was  kept  longer  it  fer- 


THE  HOP  APHIS.  / 

mented.  The  decoction  then  was  not  so  effective  an  insecticide  as  when 
fresh,  probably  because  of  a  breaking-down  of  the  nicotine  products 
under  the  action  of  fermentation.  The  kerosene  emulsion  also  should 
not  be  kept  too  long  before  using,  because  of  a  tendency  which  even 
the  most  carefully  made  emulsions  have  to  separate  and  allow  free  oil 
to  appear. 

The  spray  material  was  made  up  by  taking  forty  gallons  of  the 
tobacco  decoction  and  to  it  adding  three  and  one-half  gallons  of  the 
emulsion.    This  was  stirred  frequently  while  being  applied  to  the  vines, 


Fig.  4.    Spraying  hop  vines. 

so  that  the  mixture  was  of  uniform  quality.  We  found  that  a  certain 
amount  of  separation  of  the  oil  in  the  spray  tank  was  unavoidable  in 
practice.  To  avoid  placing  this  upon  the  vines  it  is  best  not  to  use  for 
spraying  the  last  two  or  three  inches  of  material  from  the  tank.  The 
free  oil  burns  the  hop  foliage  badly,  hence  this  residue  should  be 
emptied  out  before  refilling  the  tank. 

We  used  in  this  yard  an  outfit  consisting  of  a  fifty-gallon  barrel  and 
a  good  spray  pump  set  up  on  a  sled.  (See  Fig.  2.)  This  when  loaded 
was  not  too  heavy  for  one  horse  to  haul  quite  readily.  Two  fifty-foot 
leads  of  hose  and  short  rods  (see  Fig.  3)  were  found  to  economize 
labor  to  the  best  advantage,  the  sprayers  working  away  from  the  outfit 
and  covering  five  rows  on  each  side  of  the  one  through  which  the  horse 


8  UNIVERSITY  OF  CALIFORNIA— EXPERIMENT  STATION. 

was  driven.  Vermorel  (eddy-chamber)  nozzles  were  used  in  this  work, 
and  great  care  was  taken  to  get  the  spray  material  on  the  under  sides 
of  the  leaves,  where  most  of  the  aphids  are  found.  (See  Fig.  4.)  This 
one  outfit,  employing  one  man  to  drive  and  pump  and  two  men  spray- 
ing, covered  from  three  and  one-half  to  four  acres  a  day.  We  found  it 
necessary  to  go  over  the  yard  three  times  before  the  hops  were  ready 
to  pick,  and  we  averaged  for  these  three  sprayings  sixty  gallons  of 
material  to  the  acre  for  each  application,  or  for  the  entire  work  one 
hundred  and  eighty  gallons  to  the  acre.  To  economize  labor,  enough 
of  the  material  to  last  a  half  day  was  made  up  in  the  morning  and 
again  at  noon;  and  this  was  hauled  out  to  the  hop  yard  and  left  in  a 
situation  that  would  enable  the  men  who  were  employed  in  spraying  to 
fill  up  the  barrel  on  the  sled  with  the  least  possible  delay.  (See  Frontis- 
piece.)    "We  found  this  to  be  a  great  economizer  of  time  and  expense. 

The  cost  of  the  materials  used  was  as  follows:  Kerosene,  24  cents 
per  gallon ;  soap,  5  cents  per  pound;  tobacco  waste,  1%  cents  per  pound. 
Labor  was  worth  $1.25  a  day,  and  the  horse  50  cents  a  day.  Using 
these  figures  as  a  basis  we  find  that  each  of  the  three  sprayings  on  this 
place  cost  less  than  $2  an  acre,  or  for  the  entire  season's  work  on  the 
fifty  acres  about  $240. 

The  results  obtained  by  this  spraying  amply  justify  the  expenditure 
involved.  The  material  used  was  effective  upon  the  aphids,  and  when 
the  crop  on  this  yard  was  gathered  it  was  uninjured,  while  untreated 
vines  and  yards  in  the  neighborhood  had  the  value  of  their  product, 
both  in  quantity  and  quality,  reduced  fully  one  half. 

Whale-Oil  Soap  and  Quassia. — The  other  hop  yard  in  which  experi- 
mental and  control  work  was  done  is  six  miles  from  Watsonville,  and 
covered  nearly  one  hundred  acres.  The  aphids  did  not  spread  over  all 
this  yard,  and  it  was  necessary  to  spray  only  some  fifteen  acres  of  it. 
Work  was  begun  in  the  last  week  of  June,  as  by  that  time  the  presence 
of  the  aphids  was  quite  evident.  The  spray  material  used  was  made  up 
of  the  extract  of  quassia  chips,  and  whale-oil  soap.  The  quassia  extract 
was  made  by  soaking  the  chips  in  water  for  a  day  or  two  and  then  boil- 
ing thoroughly  for  two  hours.  Seven  pounds  of  the  chips  were  thus 
treated  in  three  gallons  of  water,  and  the  extract  obtained  was  poured 
into  two  hundred  and  fifty  gallons  of  water  in  which  nine  pounds  of 
whale-oil  soap  had  been  dissolved.  This  was  sprayed  upon  the  vines, 
the  outfit  used  and  the  method  of  application  being  about  the  same 
as  that  previously  described.  The  same  amount  of  material  per  acre  was 
used  in  spraying  this  yard  as  in  the  other  case;  that  is,  sixty  gallons 
for  each  spraying,  or  a  total  of  one  hundred  and  eighty  gallons  for  the 
season's  work. 

The  cost  of  the  materials  used  was  as  follows :  Quassia  chips,  10  cents 


THE  HOP  APHIS.  V 

per  pound;  whale-oil  soap,  7  cents  per  pound.  The  labor  and  the  ex- 
pense of  a  horse  were  the  same  as  in  the  first  described  experiment. 
Figuring  from  this  basis  we  find  the  cost  of  spraying  with  this  material 
to  be  somewhat  less  than  with  the  kerosene  emulsion  and  tobacco  decoc- 
tion, amounting  to  between  $4.50  and  $5.00  per  acre  for  the  season's 
work.  The  aphids  were  well  controlled  by  the  spraying  with  this 
material  in  this  yard,  and  no  loss  was  caused  by  them  here.  Again,  the 
near-by  hop  vines  and  yards  not  treated  had  the  value  of  their  product 
reduced  fully  one  half  by  the  aphids. 

The  experimental  work  in  these  two  yards  indicated  that  both  the 
extract  of  quassia  and  the  tobacco  decoction  were  effective  in  destroy- 
ing the  aphids.  They  did  not,  however,  spread  out  over  the  leaves  in  a 
satisfactory  manner  when  used  alone.  It  was  also  shown  that  neither 
the  kerosene  emulsion,  nor  the  solution  of  whale-oil  soap,  at  the  strength 
at  which  it  was  safe  to  use  them,  were  alone  sufficient  to  effectively  con- 
trol the  aphids.  By  combining  the  tobacco  decoction  with  the  kerosene 
emulsion,  or  the  quassia  extract  with  the  whale-oil  soap,  the  insecticidal 
value  of  both  materials  is  fully  utilized  and  the  spreading  of  the  liquid, 
so  necessary  for  satisfactory  work,  is  accomplished. 

Though  many  other  spray  materials  were  experimented  with,  none 
of  them  controlled  the  aphis  nearly  so  well  as  the  two  above  described. 

Probably  the  ease  of  obtaining  either  the  tobacco  waste  or  the  quassia 
chips,  and  the  cost  of  these  ingredients  of  the  spray  at  the  time  when 
purchased,  will  determine  which  of  the  two  materials  is  to  be  used  by 
the  hop-grower. 

In  the  treated  yard  the  yield  in  hops  in  1903  was  about  1700  pounds 
per  acre,  and  in  the  untreated  yards  the  yield  was  on  the  average  only 
900  pounds  on  the  same  area.  The  fact  that  by  treatment  such  as  has 
been  indicated  growers  have  been  able  to  double  the  returns  on  each 
acre  of  hops  amply  proves  that  careful  spraying  for  the  hop  aphis 
pays  well. 

DESCRIPTION  AND  METHOD  OF  GROWTH  OP  THE  HOP  APHIS. 

In  our  California  yards  the  first  hop  aphids  to  appear  in  the  spring, 
as  mentioned  above,  are  wingless  females,  which  produce  living  young 
parthenogenetically ;  that  is,  without  the  male  aphids  being  present. 
(See  Fig.  5.)  These  were  probably  produced  from  eggs  in  which  the 
insect  had  passed  the  winter,  and  correspond  with  what  is  called  the 
"stem  mother."  These  females  when  full  grown  are  from  one  and  one 
half  to  two  millimeters  (one  eighteenth  to  one  twelfth  of  an  inch)  in 
length.  They  vary  in  color  from  very  pale  green  to  dark  green.  They 
are  provided  with  rather  long  antennae  set  on  frontal  tubercles,  which 


10 


UNIVERSITY  OF  CALIFORNIA— EXPERIMENT  STATION. 


Fig.  5.  Wingless  partheno- 
genetic  female  hop  aphid  of 
the  first  or  spring  generation. 
Length,  1/ 18  to  1  /12  of  an  inch. 


are  toothed  internally.  The  first  joints  of  the  antennae  are  similarly 
toothed.  (See  Figs.  5,  6,  and  7.)  This  character  of  the  frontal 
tubercles  and  first  antennal  joint  serves  well  to  identify  the  species. 
The  honey-tubes  or  nectaries  which  are  located  on  the  distal  portion  of 
the  abdomen,  one  on  either  side  of  the  medial  line  of  the  dorsum,  are 
quite  prominent.  The  young  are  similar  in  appearance  to  the  adults, 
but  are  smaller.    The  mother  aphids  produce  their  young  at  the  rate  of 

from  four  to  six  a  day,  and  specimens  kept 
under  observation  by  us  continued  this  rapid 
production  of  young  for  twelve  days.  A  total 
of  sixty-six  young  were  produced  by  some  of 
these  females.  These  young  aphids  in  their 
turn  begin  to  reproduce  their  kind  on  about 
the  fourth  day  after  their  birth.  The  rapid 
increase  in  the  numbers  of  the  aphids  found 
upon  the  hop  vines  in  infested  fields  is  easily 
understood  when  we  consider  the  rapidity  of 
reproduction  shown  by  these  creatures. 

In  from  two  to  three  weeks  after  these  wing- 
less parthenogenetic  females  first  appear  winged  aphids  will  occasion- 
ally be  found.  These  differ  from  the  first  chiefly  in  having  two  pairs 
of  relatively  large  wings,  which  are  quite  delicate  in  structure  and 
almost  transparent.  The  fore  wings  are  much  larger  than  the  hind 
wings  and  both  are  rather  sparsely  veined  in  a  manner  characteristic 
of  the  group.  These  winged  aphids,  which  produce  living  young  in 
the  same  manner  as  do  the  wingless  ones, 
first  described,  fly  sufficiently  well  to  spread 
the  attack  to  other  vines.  Thus  in  a  very 
short  time  what  was  at  first  a  local  infesta- 
tion of  small  moment  may  become  an  attack 
involving  large  areas  of  the  hop  yard.  The 
winged  parthenogenetic  females  continue  to 
be  developed  in  about  the  proportion  of  one 
to    a    hundred   wingless   individuals   until 

about   picking  time.  Fig.  6.     Wingless    female    hop 

aphid.    Late  summer  generation. 
Much  magnified. 

Autumnal  H istory.— After  the  hops  were 
picked  and  the  pistillate  plants  were  practically  all  dead,  above  ground, 
the  male  plants  were  kept  under  observation  for  three  weeks.  The  lice 
continued  to  develop  and  increase  materially  in  numbers  upon  these 
plants,  yet  only  an  occasional  winged  specimen  was  developed,  as  was 
true  during  the  summer,  and  these  were  all  parthenogenetic  females. 
(See  Fig.  6.)  By  the  end  of  August  or  early  in  September  the  male 
hop  plants  were  also  all  quite  dead  above  the  ground  and  in  many 


THE  HOP  APHIS.  11 

instances  the  yards  had  been  closely  pastured  oft'  by  sheep.  Before  this 
had  occurred,  however,  an  occasional  wingless  oviparous  female  aphid 
was  observed,  and  also  a  number  of  winged  males:  and  these  were 
always  found  on  the  lower  leaves  and  runners  of  the  male  hop  vine. 

These  egg-laying  females  differ  in  appearance  but  slightly  from  the 
females  previously  described,  yet  on  examination  under  the  microscope 
the  eggs  in  them  can  be  quite  readily  discerned.  The  winged  males  are 
the  offspring  of  wingless  parthenogenetic  mothers.  These  differ  from 
those  aphids  previously  described  in  being  rather  slimmer  and  longer, 
and  in  being  marked  with 
black  dorsally;  the  whole 
color-effect  of  the  bodies  of 
these  males  being  darker  than 
is  the  case  with  the  others. 
(See  Fig.  7.) 

We  were  unable  to  actually 
find  the  eggs  of  the  hop  aphis 
in  any  of  the  yards  under 
observation,  nor  upon  any  of 
the     neighboring     vegetation, 

Fig.  7.    Winged  male  hop  aphid.    Much  magnified. 

but    the    wingless    egg-laying 

females  being  produced  on  the  male  hop  vine  only  is  good  presumptive 
evidence  that  the  eggs  are  deposited  there.  The  male  hop  plants  on 
which  the  aphids  were  first  found  in  1903  were,  in  the  majority  of  cases, 
the  same  plants  on  which  they  were  found  at  the  end  of  the  preceding 
season,  and  in  the  few  cases  where  these  last  infested  vines  were  not  the 
same  that  were  first  infested  they  were  near  neighbors  of  these  vines. 
Our  observations  have  been  continued  into  the  present  year,  1904,  and 
the  same  holds  true,  the  same  vines  being  again  the  first  to  show  the 
presence  of  the  hop  aphis,  and  from  them  the  trouble  has  again  spread. 

Wintering.— The  California  data  given  above  seem  to  indicate  that 
the  hop  aphis  hibernates  in  some  situation  in  the  hop  yard  itself  in  the 
egg  form.  The  eggs  may  be  placed  either  upon  the  cut  stalks  of  the 
hop  vines  at  or  just  beneath  the  surface  of  the  ground,  or  upon  the 
roots  of  the  plant  or  even  in  the  ground  contiguous  to  the  vines.  At 
no  time  have  we  been  able  to  find  any  evidence  of  the  presence  of  the 
hop  aphis  on  any  other  vegetation  near  to  or  in  the  hop  yards.  This 
fact,  coupled  with  their  appearing  first  upon  the  same  male  plants 
year  after  year,  or  at  least  in  the  same  parts  of  the  yards,  and  their 
continuance  upon  these  plants  until  the  very  end  of  the  season,  points 
strongly  to  the  probability  of  hibernation  taking  place  in  the  suggested 
situations.     That  wingless  oviparous  females  and  winged  males  are  to 


12  UNIVERSITY  OF  CALIFORNIA— EXPERIMENT  STATION. 

be  found  upon  these  late-growing  male  vines,  and  that  these  are  the 
iast  forms  of  the  aphids  to  be  produced  in  the  hop  yards,  would  seem 
to  add  weight  to  the  idea  of  hibernation  occurring  in  the  egg  form  in 
the  hop  yard  itself. 

History  Elsewhere.— The  insect  has  an  entirely  different  history  else- 
where, and  gave  rise  to  many  conflicting  theories  by  students  of  this 
interesting  insect  before  the  full  life-history  was  worked  out.  Probably 
the  most  complete  study  that  has  been  made  was  published  by  the  late 
Prof.  C.  V.  Riley,  then  entomologist  of  the  United  States  Department 
of  Agriculture.  This  account  appears  in  the  report  of  that  department 
for  1888.  Briefly  this  life  history  as  given  by  Professor  Riley  is  as 
follows :  ' '  Hibernation  takes  place  on  different  varieties  and  species 
of  prunus  (plum),  and  the  little,  glossy,  black,  ovoid  eggs  of  the  species 
are  found  attached  to  the  terminal  twigs,  and  especially  in  the  more  or 
less  protected  crevices  around  the  buds.  From  this  winter  egg  there 
hatches  a  stem-mother,  which  is  characterized  by  being  somewhat 
stouter,  with  shorter  legs  and  honey-tubes  than  in  the  individuals  of  any 
other  generation.  Three  parthenogenetic  generations  are  produced 
upon  primus,  the  third  being  winged.  This  last  is  *  *  *  called 
the  migrant,  and  it  instinctively  flies  to  the  hop  plant,  which  is  entirely 
free  from  attack  during  the  development  of  the  three  generations  upon 
the  plum.  A  number  of  parthenogenetic  generations  are  produced 
upon  the  hop  until  in  autumn,  and  particularly  during  the  month  of 
September,  winged  females  are  again  produced.  This  is  the  return 
migrant,  and  she  instinctively  returns  to  the  plum.  Here  she  at  once 
settles  and  in  the  course  of  a  few  days,  according  as  the  weather  per- 
mits, produces  some  three  or  more  young.  These  are  destined  never 
to  become  winged  and  are  true  sexual  females.  Somewhat  later  on  the 
hop  the  true  winged  male,  and  the  only  male  of  the  whole  series,  is 
developed,  and  these  males  also  congregate  upon  the  plum,  on  the  leaves 
of  which  toward  the  end  of  the  season  they  may  be  found  pairing  with 
the  wingless  females,  which  stock  the  twigs  with  the  winter  eggs. ' ' 

Possibilities  of  Winter  Treatment. — Professor  Riley  suggests  that  if 
the  eggs  of  the  species  upon  the  plum  trees  were  treated  by  sprays 
strong  enough  to  kill  them  the  hops  could  be  thoroughly  protected.  The 
conditions  that  appear  to  exist  in  the  hop-growing  regions  of  California, 
as  detailed  above,  are  such  that  this  method  of  control  does  not  offer 
any  promise  of  success.  These  field  studies  during  the  past  two  years 
have  shown  that  here  the  life-history  as  given  by  Professor  Riley  is 
not  at  all  followed  out,  since  careful  search  of  plum  trees,  both  near  to 
and  distant  from  hop  yards,  has  failed  at  all  times  with  us  to  disclose 


THE  HOP  APHIS.  13 

the  presence  of  the  hop  aphis  (Phorodon  humuli)  or  its  eggs.  We  are 
forced  to  conclude  that,  probably  because  of  climatic  conditions,  this 
aphis  has  in  this  State  a  life-history  much  at  variance  with  that  given 
by  Professor  Riley  and  generally  accepted  as  normal  with  the  species. 

It  does  not  seem,  therefore,  that  in  this  State  any  treatment  of  trees 
of  the  varieties  of  prunus  (plums,  prunes,  green  gages,  etc.)  for  the 
destruction  of  the  hibernating  hop  aphids,  or  their  eggs,  would  be  of 
value,  since  they  are  not  there  to  be  destroyed.  The  treatment  of  the 
soil  of  the  hop  yards  during  the  winter  with  the  idea  of  destroying  the 
possible  aphid  eggs  might  accomplish  more,  but  we  do  not  think  that 
there  is  much  ground  for  hope  of  success,  because  of  the  practical  diffi- 
culty of  making  such  a  treatment  thorough  enough  to  be  effectual. 

RESUME. 

To  sum  up,  then,  it  seems  that  under  our  California  conditions  the 
hop  aphids  appear  first  in  the  spring  as  wingless  parthenogenetic 
females  (stem-mothers)  upon  the  staminate  (male)  vines  (p.  3). 
They  reproduce  rapidly  here  (p.  4),  and  occasional  winged  indi- 
viduals soon  appear  (p.  4).  These  migrate  to  near-by  vines,  and  the 
aphids  are  thus  spread  through  the  affected  yard  (p.  4).  They  confine 
their  attack  to  the  leaves  of  the  vine  until  young  hop  cones  are  formed, 
and  then  attack  these  also  (p.  4).  The  damage  done  is  twofold,  con- 
sisting of  a  reduction  in  the  size  and  weight  and  also  a  loss  of  value  to 
the  hops,  because  of  the  poor  aroma,  due  to  the  actual  presence  of  the 
aphids  in  them  (p.  5).  Experiments  with  sprays  made  up  of  mixtures 
of  kerosene  emulsion  and  tobacco  decoction  (p.  5)  and  of  whale-oil 
soap  and  quassia  extract  (p.  8)  were  entirely  effective  in  controlling 
the  aphids  (p.  9).  Other  materials  experimented  with  were  not  nearly 
so  satisfactory  in  their  effect  as  were  the  two  described  (p.  9).  The 
hop  aphis  is  quite  readily  identified  (p.  9).  The  wingless  and  winged 
parthenogenetic  females  (p.  10)  are  followed  late  in  the  year  by  wing- 
less oviparous  females  and  winged  males  (p.  10).  These  are  always 
found  upon  the  late-growing  male  vines  and  in  no  other  situations 
(p.  11).  From  the  evidence  at  hand  it  would  seem  that  hibernation  of 
the  aphids  took  place  in  the  egg  form  in  the  hop  yards  (p.  11).  This 
does  not  agree  with  the  history  of  the  insect  elsewhere  as  published  by 
Professor  Riley  (p.  12),  and  there  seems  to  be  no  possibility  of  a  suc- 
cessful winter  treatment  for  the  hop  aphis  under  our  California  condi- 
tions (p.  13).  Indeed,,  spraying  with  either  of  the  washes  described 
in  this  paper  (pp.  5  to  9)  so  completely  controls  the  hop  aphis  that 
the  insect  need  not  be  considered  a  menace  to  the  crop  (p.  9)  and  the 
necessity  of  winter  spraying  is  removed. 


REPORTS   AND  BULLETINS  AVAILABLE  FOR  DISTRIBUTION. 


REPORTS. 

1896.  Report   of   the    Viticultural    Work   during   the   seasons    1887-93,    with    data 

regarding  the  Vintages  of  1894-95. 

1897.  Resistant   Vines,   their   Selection,   Adaptation,   and   Grafting.     Appendix  to 

Viticultural  Report  for  1896. 

1898.  Partial  Report  of  Work  of  Agricultural   Experiment   Station  for  the  years 

1895-96  and  1896-97. 
1900.     Report  of  the  Agricultural  Experiment  Station  for  the  year  1897-98. 
1902.     Report  of  the  Agricultural  Experiment  Station  for  1898-1901. 

BULLETINS. 
No.  121.     The  Conservation  of  Soil  Moisture  and  Economy  in  the  Use  of  Irrigation 
125.     Australian   Saltbush.  [Water. 

127.  Bench-Grafting  Resistant  Vines. 

128.  Nature,  Value,  and  Utilization  of  Alkali  Lands. 

129.  Report  of  the  Condition  of  Olive  Culture  in  California. 

131.  The  Phylloxera  of  the  Vine. 

132.  Feeding  of  Farm  Animals. 

133.  Tolerance  of  Alkali  by  Various  Cultures. 

134.  Report  of  Condition  of  Vineyards  in  Portions  of  Santa  Clara  Valley. 

135.  The  Potato-Worm  in  California. 

136.  Erinose  of  the  Vine. 

137.  Pickling  Ripe  and  Green  Olives. 

138.  Citrus  Fruit  Culture. 

139.  Orange  and  Lemon  Rot. 

140.  Lands  of  the  Colorado  Delta  in  Salton  Basin,  and  Supplement. 

141.  Deciduous  Fruits  at  Paso  Robles. 

142.  Grasshoppers  in  California. 

143.  California  Peach-Tree  Borer. 

144.  The  Peach- Worm. 

145.  The  Red  Spider  of  Citrus  Trees. 

146.  New  Methods  of  Grafting  and  Budding  Vines. 

147.  Culture  Work  of  the  Substations. 

148.  Resistant  Vines  and  their  Hybrids. 

149.  California  Sugar  Industry. 

150.  The  Value  of  Oak  Leaves  for  Forage. 

151.  Arsenical  Insecticides. 

152.  Fumigation  Dosage. 

153.  Spraying  with  Distillates. 

154.  Sulfur  Sprays  for  Red  Spider. 

155.  Directions  for  Spraying  for  the  Codling-Moth. 

156.  Fowl  Cholera. 

157.  Commercial  Fertilizers. 

158.  California  Olive  Oil ;  its  Manufacture. 

159.  Contribution  to  the  Study  of  Fermentation. 

CIRCULARS. 

No.  1.  Texas  Fever.  No.  8.  Laboratory     Method     of     Water 

2.  Blackleg.  Analysis. 

3.  Hog  Cholera.  9.  Asparagus  Rust. 

4.  Anthrax.  10.  Reading    Course    in     Economic 

5.  Contagious  Abortion  in  Cows.  Entomology. 

6.  Methods  of  Physical  and  Chem-  11.  Fumigation  Practice. 

ical  Soil  Analysis.  12.     Silk  Culture. 

7.  Remedies   for   Insects. 

Copies  may  be  had  by  application  to  the  Director  of  the  Experiment 
Station,  Berkeley,  California. 


